Cryopumps currently available, whether cooled by open or closed cryogenic cycles, generally follow the same design concept. A low temperature second stage cryopanel array, usually operating in the range of 4-25 K, is a primary pumping surface. This surface is surrounded by a high temperature radiation shield usually operated in the temperature range of 40-130 K, which provides radiation shielding to the lower temperature array. The radiation shield generally comprises a housing which is closed except at a frontal cryopanel array positioned between the primary pumping surface and the chamber to be evacuated. This higher temperature, first stage, frontal array serves as a pumping site for high boiling point gases such as water vapor, known as Type I gases.
In operation, high boiling point gases such as water vapor are condensed on the frontal array. Lower boiling point gases pass through the frontal array and into the volume within the radiation shield. Type II gases, such as nitrogen, condense on the second stage array. Type III gases, such as hydrogen, helium and neon, have appreciable vapor pressures at 4K. To capture Type III gases, inner surfaces of the second stage array may be coated with an adsorbent such as activated carbon, zeolite or a molecular sieve. Adsorption is a process whereby gases are physically captured by a material held at cryogenic temperatures and thereby removed from the environment. With the gases thus condensed or adsorbed onto the pumping surfaces, only a vacuum remains in the work chamber.
In cryopump systems cooled by closed cycle coolers, the cooler is typically a two stage refrigerator having a cold finger which extends through the radiation shield. The cold end of the second, coldest stage of the refrigerator is at the tip of the cold finger. The primary pumping surface, or cryopanel, is connected to a heat sink at the coldest end of the second stage of the cold finger. This cryopanel may be a simple metal plate, a cup or an array of metal baffles arranged around and connected to the second stage heat sink as, for example, in U.S. Pat. Nos. 4,555,907 and 4,494,381, which are incorporated herein by reference. This second stage cryopanel may also support low temperature condensing gas adsorbents such as activated carbon or zeolite as previously stated.
The refrigerator cold finger may extend through the base of a cup-like radiation shield and be concentric with the shield. In other systems, the cold finger extends through the side of the radiation shield. Such a configuration at times better fits the space available for placement of the cryopump.
The radiation shield is connected to a heat sink, or heat station, at the coldest end of the first stage of the refrigerator. This shield surrounds the second stage cryopanel in such a way as to protect it from radiant heat. The frontal array which closes the radiation shield is cooled by the first stage heat sink through the shield or, as disclosed in U.S. Pat. No. 4,356,701, which is incorporated herein by reference, through thermal struts.
Early frontal arrays comprised circular louvers mounted on thermal rods coupled to the radiation shield. Certain louvers may be in the form of chevrons to be more opaque to radiation.
Other pump designs, such as the pump described in U.S. Pat. Nos. 4,449,373, 4,611,467 and 5,211,022, which are incorporated herein by reference, replace the louvers of the first stage with a plate having multiple orifices. The orifices restrict the flow of gases to the second stage array compared to the chevrons or louvers. In certain applications like sputtering processes, by restricting flow to the inner second stage pumping area, a percentage of inert gases are allowed to remain in the working space to provide a moderate pressure (typically 10−3 Torr or greater) of inert gas for optimal processing. However, higher condensing temperature gases, such as water, are promptly removed from the environment by condensation on the frontal orifice plate.
The frontal array protects the second stage array to reduce radiant heat from striking the second stage, to control Type II and III gas flow rates to the second stage array, and to prevent Type I, higher boiling point temperature, gases from condensing on the colder surfaces and any adsorbent layer. The reduction in radiation and flow rates lowers the temperature of the second stage cryopanel surfaces and the condensed gases on these surfaces as well as any adsorbent. The lower temperature results in an increased gas capture capacity and reduces the frequency of regeneration cycles. The louvers provide very good radiation shielding as compared to the orifice plates, which contain orifices that provide direct line of sight of the radiant heat to the second stage cryopanel surfaces. However, orifice plates severely restrict Type II and Type III gases to the second stage cryopanels compared to the louvers, which results in lower pumping speeds for these gases. In some applications, this severe restriction of pumping speed is preferred because a percentage of inert gases are allowed to remain in the working space of the process chamber to provide a moderate pressure of inert gas for optimal sputtering or other processing.
A modified orifice (sputter) plate is disclosed in published U.S. application 2013/0312431, incorporated herein by reference in its entirety. That frontal orifice plate has a plurality of orifices, each orifice having a flap that is bent from and attached to the frontal plate at an edge of the orifice, and each flap is arranged in a path that passes through the frontal plate. The orifices may be rectangle shaped, square shaped, trapezoid shaped, circle shaped, triangle shaped, or any other shape. The flaps are preferably bent at an angle between 10° and 60° relative to the surface of the frontal baffle plate, and most preferably are bent at an angle between 25° and 35°. For greater speed but higher heat load on the second stage array, angles of 35-45° are preferred. The flaps serve as baffles, so the plate has also been termed a baffle plate.
Advantages of a cryopump having the frontal baffle plate include simplicity of manufacturing and improved blocking of radiation from a process chamber to which the cryopump is attached. Another advantage of a cryopump having the frontal baffle plate is improved distribution of the Type II gases and Type III gases at the second stage array of the cryopump.